Avoid Itertools::flatten to support rust 1.29
Rust 1.29 stabilized Iterator::flatten which now conflicts with
Itertools::flatten. Use .flat_map(|x| x) instead as a workaround
to bide time until Itertools upstream resolves this conflict.
Rust 1.29 stabilized Iterator::flatten which now conflicts with
Itertools::flatten. Use .flat_map(|x| x) instead as a workaround
to bide time until Itertools upstream resolves this conflict.